HTC root

search root HTC D816v in google and you will get a guide

https://forum.xda-developers.com/showthread.php?t=2775801

other pages may help

https://community.htc.com/tw/chat.php?mod=viewthread&tid=1508

http://lbear.pixnet.net/blog/post/48931096

if you are using mac, you may need 

https://www.android.com/filetransfer/

to transfer file from mac to android